Learning Forum

[ciciola.2027298]

Hi everyone,

I'm meshing a 3D complex geometry in Ansys Fluent Meshing. I applied local sizing, setting the face size to 0.002, and I imposed the first height of the boundary layer at a value of 0.0007. So, I expected the Aspect Ratio to be around 2 or 3. However, after generating the volume mesh, I obtained an Aspect Ratio of the surface mesh of over 20,000, and the Aspect Ratio of the volume mesh is around 100. Orthogonality and skewness are approximately 0.3 and 0.6, respectively.

Why is my Aspect Ratio so high?

[Rob]

Did you set the whole, and all face size at 0.002? Also, if you have any proximity/curvature functions active you could have smaller/larger surface facets due to the geometry. If you plot the poor cells where are they?

[ciciola.2027298]

the cells with highest aspect ratio are near the geometry, a lot of them are in the boundary layer (I'm sorry i can't show you any images) and this is understandable. But, I can't explain the high value of aspect ratio and, also, the high value of Skewness (it's over 1.2)

[Rob]

Skew can't exceed 1.0 as it's 0 (perfect) to 1.0 (flat), so please check. 

Re the aspect ratio. Think what the purpose of a boundary layer mesh is. So, I'd expect a high aspect ratio. This may be acceptable, but assumes the flow values don't change much (and do so slowly) along the inflated surface. 

[ciciola.2027298]

the first photo below is the summary of Diagnostic Tool after generating surface mesh. 

So, after generating the volume mesh, I clicked on Summary of Diagnostic Tool and Fluent printed this below

[ciciola.2027298]

It happens only when I use Poly-hexcore cells, not when I use tetrahedral, but in the latter case, Fluent generates too much cells

[Rob]

Have you got highly curved surfaces either in close proximity or that are poorly resolved? That looks like a poly cell (possibly poly-prism) has got turned inside out.  

[ciciola.2027298]

Yes, the geometry is complex (It's the upper part of a motorbike). How can I find and fix the problem?